Tucson’s population has more than doubled since the 1960s. The city’s footprint has also swelled, now sprawling far into the surrounding desert. But the Tucson area has lost more than saguaro habitat—the city has slowly nibbled away at the riparian zones along the Santa Cruz River (running along the west side of the photos) and Rillito River (running along the north side). Take a look.

August 22, 1965, taken by Gemini V astronauts Gordon Cooper and Pete Conrad.

October 28, 2011, taken by Landsat 5.

The loss of that habitat is significant—riparian zones are often much more productive than their surroundings, especially so in deserts. The question that’s bugging me is, why did Tucson gobble up that important habitat and not the surrounding desert (of which there is plenty)?